In completing oil and gas wells in unconsolidated or loosely consolidated formations, it has been found necessary to "gravel pack" such formations by pumping downwardly into the well a mixture of fluid and aggregate sometimes referred to as a "sand slurry". Typically, the slurry of gravel or aggregate in a liquid carrier is pumped into the annular space between the formation and a liner in an area aligned with the production zone of the well. The gravel packing may also be pumped into the cylindrical area within a perforated casing in which a well screen is positioned in order to maintain the integrity of oil and gas flow through the production formation and into the casing.
Typically in offshore operations, it has been necessary to perform such gravel packing from a platform having well treatment tubing extending from the platform downwardly through the wellhead and into the well. In the case of a subsea well positioned some distance away from a platform, such treatment normally requires a floating service vessel to be positioned above the well for vertical re-entry.
The avoidance of vertical re-entry has been accomplished in many subsea oil well servicing and completion operations by the utilization of through-the-flowline (TFL) tools. Typically, TFL tools require a particular configuration of platform equipment including a hydraulic tool stuffer (horizontal lubricator) designed to admit the TFL tool into a line that may be under pressure. The tool is then pumped downwardly through tubing to a subsea christmas tree designed to guide the tool smoothly from the flowline or tubing into the wells' tubing string. The well tubing may be a dual string tubing completion having one or more H-member type crossovers to allow the TFL tool to be circulated downwardly through one of the tubings of the dual string, through the H-member crossover and then recovered by reversing the direction of flow through the other tubing of the dual string.
The utilization of such TFL pump down techniques allow satellite wells to be serviced from a platform not positioned over those wells by the use of interconnecting tubing or flowlines for delivering and returning the TFL tool from the satellite well to the platform. Typical through-the-flowline operations utilizing TFL tools include paraffin scraping; downhole equipment service such as storm choke service; gas lift installation; bottom hole pressure measurements; and, workover operations including eliminating sand bridges and sand washing. Insofar as known, through-the-flowline operations have not been used to gravel pack a well.